I always wonder - the machines on the museum look mint, like the fabulous 9100 or the gorgeous 9825. I suppose the machines were cleaned and failures on the finish repaired, maybe even with a new layer of paint? Even though I disassemble my machines as far as possible, in order to put them in a cleaning bath, I don't get the same results. And there are limits: A keyboard like the one in the 97 cannot be disassembled key by key (or am i wrong?), which means the whole plastic assemble must be washed. Worse on the 9810 - or am i wrong again? Please reveal your secret cleaning tips!

There is a trick involved: the machines *are* mint. The machines have been bought by universities (money didn't matter a lot in the 70s) and quickly been stored away, due to faster, better and cheaper machines being available. They only did serve a couple of hours by then - which means "no wear". Depending on the storage conditions they will stay in this condition for 30 years :)

After all, it doesn't make a lot of sense to "rework" a calculator, as mint ones are available easily (but being HP, not for cheap).
When you really want to clean them, try water & soap and a soft sponge. For some dirt, window cleaner works great (this is what I use most of the time); you might want to try alcohol based cleaners as well (the more aggressive, the greater the chance to damage the surface - better try on an invisible part of the machine). For really tough dirt, aceton does a great job - it will most likely damage plastic parts though.

I would not recomend dismantling a 97 (or other Topcat) keyboard. The 'hats' don't normally fit so well the second time round.. If you must, then after removing the PCB, contact sheets, etc, press on the bottom of each key stem with something like a screwdriver handle and remove the hat, key and spring. When reassembling, use a hot soldering iron to form over the end of any stem that doesn't hold that well.

To dismantle a 9100 keyboard, take off the baseplate and bezel, then remove the PCB (note where the screws go, some of the holes are used to hold the baseplate on). Remove the toggle switches (2 screws each). Loosen the nut on the decimal selector switch, slip that out of the frame. You can now separate all the electronic bits from the casing. Use a small screwdriver from inside the casing to free the locking hook on each key, slide out the key complete with the metal pin (don't try to separate those). When reassembling, put a new 'size 006' O-ring on each pin. They were fitted there originally, but they have long since cracked and fallen off in most cases.

The 9810 keyboard comes apart very easily. After removing the complete assembly from the machine, pull off the interconnect PCB at the side (this connects the encoder board to the key matrix). Disconnect the edge connector that connects the wiring harness to the encoder board, then undo the screws and take off the encoder. Undo the 6 screws that hold the keyboard to the bezel, take it off. If you want, you can remove the display window with the obvious screws. Back on the key chassis, take off the front strip by undoing the 6 larger screws. Then remove the mains switch (2 obvious screws). The keys just pull out complete with their plungers and springs -- you can separate the key from the plunger if you want to. Remove the screws and take off the PCB, then remove the key housings. Many of the screws in this assembly were fitted with 'threadlock'. I find it a good idea to run a tap (4-40 UNC) into all the holes and to run the screws into a die (same thread). Makes reassembling a lot easier.

The 9830 is somewhat similar. Unplug the interconnect PCB (which carries the wiring harness in this machine). Undo the screws and take off the encoder board. Then remove the bezel. Before removing the front strip, desolder the wires to the power-on lamp from the keyboard PCB. Remove the front strip, then the mains switch. The keycaps pull off, but if you want to remove the switches or the PCB, you have to desolder them. I don't normally do this!

If you must dismantle an HP chicklet keyboard (9815, original 9825, 9831, etc), remove the logic boards, etc. Then undo all the tiny screws on the back and pull off the keyboard PCB. Make sure you have a diagram of the key positions before doing this, some _will_ fall out. Recover the switch contacts, etc (like the slide switches on a handheld. When reassembling, put the casing on the edge of a table/board so that the key holes are over the table. You can then fiddle the keys into place and refit the PCB.

Yes, this is by far the easiest way. Most of my machines have been dusted (brush/compressed air) and some have been gently washed in some areas with a soft cloth and soap and water. I don't dissemble, soak, repaint etc.

Some historians think that even the cleaning that I do is excessive. I think it's OK, but repainting etc. is out of the question in my opinion. Besides the hole historical issue, there's the practical question of whether the new paint will age the way old paint would over the next couple of decades. Be very careful about cleaning keypads. It's easy to put a shine on textured matt plastic.

I had the benefit of acquiring most of my calculators before ebay existed or at least was well-known. Over the years, I've seen some of my local equipment providers raise their prices by a factor of more than one hundred the day after they browsed ebay. But I still get lucky now and then.

There is a trick to cleaning the plastic calculators it's Novus Plastic Polish. Use #2 to get out light scratches and/or #1 to clean it, but don't use #3 it's too abrasive. Anyway this stuff can make a calculator look brand new again assuming that it has no physical damage or paint issues.

But like you suggest, for best results it's necessary to remove the keys, display lens, etc. and clean them separately from the rest of the case.